Imaging system in which the size and centering of the raster are kept constant



IMAGING SYSTEM IN Filed Feb. 28, 1964 D. B. SHAW RASTER ARE KEPTCONSTANT WHICH THE SIZE AND CENTERING OF THE 4 Sheets-Sheet 1 \a 8Q H I7o\& l \Ea A {:EW'Z TELEVISION BANDPASS M A RECEIVERV AMPLIFIER I9 :8 J20B 2 I 7| I IZZI ENVELOPE DEFLECTION DEFLECTION DETECTOR AMPLIFIERAMPLIFIER 12 I, 5Q 7223 DIFFER- V SWEEP swEEP ENTIATOR GENERATORGENERATOR 73 I I BISTABLE MULTI- (50 VIBRATOR L.. SYNCHRONIZING CIRCUITI HOR. HOR. l HOR ST. END SIZE ENAB. ENAB. I I I REF. 1 VERT. END ENAB.I I I I VVERT. SIZE REE l .,-VERT. s'r. ENAB. I cas w e 38 1-b c-'- 42}-b e- 45 b I I AND AND AND AND MA MM Il d- I as 46 1 I 1 g INVERTER 3g5 INVERTER I I I I I I 36W |40 43 47 I I ADDER ADDER v ADDER ADDER g I-h -k -h -k I I 37 4| r44 r48 I INTEGRATOR INTEGRATOR INTEGRATORINTEGRATOR I L :L I

e|- s2- s3- s4 D. s. SHAW 3,389,294 IMAGING SYSTEM IN WHICH THE SIZE ANDCENTERING OF THE June 18, 1968 RASTEIR ARE KEPT CONSTANT 4 Sheets-Sheet2 Filed Feb. 28, 1964 FIG. 2b

FIG. 20.

BINARY COUNTER DIODE LOGIC vn- 3 23m 555. Em U vmv 3 522m 02w Em; m 3 HEH5 .Ew I? 3 322m 6 Em; vm- E 025 55% .Ew

BINARY COUNTER DIODE LOGIC OSCILLATOR FIG. 9

I I I I l I I I I- June 18, 1968 IMAGING SYSTEM IN WHICH THE SIZE ANDCENTERING OF THE RASTER ARE KEPT CONSTANT Filed Feb. 28, 1964Hllllllllllllllllill IHIIHIIIIIHIIII llllllllllllllllll "l ""l' i"""lHmm Hllllllllllllllllill D. B. SHAW 4 Sheets-Sheet 5 '1 ID 2' LL 8 Q 2'LL.

.1: r m 2' LL June 18, 1968 D. B. SHAW 3,389,294

IMAGING SYSTEM IN WHICH THE SIZE AND CENTERING OF THE RASTER ARE KEPTCONSTANT Filed Feb. 28, 1964 4 Sheets-Sheet 4- United States Patent3,389,294 IMAGING SYSTEM IN WHICH THE SIZE AND CENTERING OF THE RASTERARE KEPT CONSTANT David B. Shaw, White Plains, N.Y., assignor toHazeltine Research Inc, a corporation'of Illinois Filed Feb. 28, 1964,Ser. No. 348,081 14 Claims. (Cl. 315 19) ABSTRACT OF THE DISCLOSURE Animaging system, such as a TV type camera, for converting anelectromagnetic image into an electrical signal representative of theimage by converting the electromagnetic image into an electrical imageand sequentially scanning the electrical image with a raster. Theelectrical image is bordered by a reticle in the form of a series offine lines or solid bands either imaged thereupon or etched onto thetarget. The reticle is scanned with the electrical image representativeof the electromagnetic image and the portion of the electrical signalwhich results from scanning the reticle is separated from the remainderof the signal by frequency or time separation. The extent of overlappingon each side of the raster is determined and compared with a signalrepresentative of the desired extent of overlapping in order to regulatethe size of the raster. The extent of overlapping along one side of theraster is compared with the extent of overlapping along the oppositeside of the raster in order to control the centering of the raster.

The present invention relates to an imaging system in which anelectromagnetic image is converted to an electrical signalrepresentative of the image. More particularly, the present inventionrelates to such a system in which the size and centering of the scanningraster used to perform the aforementioned conversion are maintainedconstant.

One type of imaging system heretofore proposed utilizes electricalfeedback to stabilize the deflection signals used to develop thescanning raster. It is assumed therein that by stabilizing thesedeflection signals the size and centering of the raster will bemaintained constant. The image tube, however, is not included within theelectrical feedback loop. Such an omission may cause the size and thecentering of the raster to vary, even though the deflection signals arestabilized. This results from the fact that associated with the imagetube are certain electrical and electromechanical parameters which aresubject to variations independent of the variations in the deflectionsignalsthe voltages applied to the accelerating and deceleratingelectrodes of the image tube, for example. If, in addition, the rasteris developed by electrostatic deflection techniques, any initialmisalignment or any subsequent shifting of one plate of a deflectionpair with respect to the other plate of that pair may also cause thesize and centering of the raster to vary. The substitution of magneticdeflection techniques for electrostatic deflection techniques to developthe raster would not simplify matters any since temperature changes inthe deflection coils of the deflection mechanism may also cause the sizeand centering of the raster to vary. Even if the image tube wereincluded within the electrical feedback loop, there 3,389,294 PatentedJune 18, 1968 'ice would be no certainty that the size and centering ofthe raster would remain constant. This is so because the stabilizationof the electricalrdeflection signals depends upon electrical referenceswhich, in turn, may themselves vary.

It will be apparent from the foregoing discussion that the diflicultiesencountered with prior art imaging systems result from their dependenceon electrical references which are not constant but which are capable ofvarying. As a result of these variations, the electrical signal producedby the system may not be an accurate representation of theelectromagnetic image.

It is an object of the present invention, therefore, to provide a newand improved imaging system which maintains the size and centering ofthe scanning raster constant, independent of all parameter variationsboth physical and electrical.

It is another object of the present invention to provide such an imagingsystem which is based on a physical, rather than an electrical,reference.

Thus, in accordance with the present invention, there is provided animaging system which comprises: means for converting an electromagneticimage into an electrical signal representative of the image byconverting the electromagnetic image into an electrical image and bysequentially scanning the electrical image; means for supplyingdeflection signals to the converting means to develop a raster used toperform the sequential scanning, the raster so developed having at leastone dimensional characteristic which may vary undesirably; means forproviding as part of the electrical image a frame in the form of areticle bordering desired areas of the electrical image, the frame beingadapted to be in an overlapping relationship with at least one side ofthe raster; means for deriving from the electrical signal representativeof the image electrical signals indicative of the extent of overlappingof the frame by the raster; comparison means utilizing the signalsindicative of the extent of overlapping of the frame by the raster fordeveloping a plurality of control signals representative of thevariations of the raster; and means for coupling the control signals tothe deflection signal supply means to prevent the variations so as toprovide accurate scanning of the desired areas of the electrical imageby the raster.

For a better understanding of the present invention together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawings, and itsscope will be pointed out in the appended claims.

Referring to the drawings:

FIG. 1 is a block diagram of an imaging system constructed in accordancewith the present invention;

FIGS. 20 and 2b show two forms of optical reticles which may be used inassociation with the image tube of the imaging system of FIG. 1 toproduce an electrical signal indicative of the variations in the sizeand centering of the raster;

FIGS. 3-5 are a series of signal waveforms helpful in understanding theoperation of the imaging system when using the reticle shown in FIG. 2a;

FIGS. 6-8 are a series of signal waveforms helpful in understanding theoperation of the imaging system when using the reticle shown in FIG. 2b;and

FIG. 9 is a block diagram of a synchronizing circuit used in the imagingsystem of FIG. 1.

3 Description and operation of the imaging system of FIG. 1

Although the teachings of the present invention are applicable in manydifferent types of imaging systems which utilizes a scanning raster toproduce an electrical signal from an electrical image, the imagingsystem will be described as it would be used in a television typedisplay system, an environment in which it is particularly useful. Insuch an environment, the imaging system represents a television pickupdevice, or camera tube, and its associated electrical circuitry whichtogether convert an optical image into an electrical signalrepresentative of the image. The electrical signal is then communicatedto a television receiver which recreates the optical image from theelectrical signal received and which displays that image on the face ofa picture tube or other display device.

Referring now to FIG. 1, the imaging system there represented includesimage sensor means for converting an optical image into a store-delectrical image which when sequentially scanned produces an electricalsignal in response thereto. Means 10 includes an image tube 11 having anelectron gun 12, a photosensitive target 13, a target ring 14,electrostatic deflecting plate pairs 15 and 16, an acceleratingelectrode 17 connected to a source of operating potential B and adecelerating electrode 18 connected to a source of operating potential BThe electron beam produced by the electron gun 12 is directed byelectrodes 17 and 18 towards the target 13, upon which is stored theelectrical image. Image tube 11 is specifically shown in FIG. 1 as avidicon, although, in general, any type of sequentially scanned imagetube may be used.

The imaging system also includes means 20 for supplying deflectionsignals to image tube 11 to develop a raster from the electron beam.Means 20 includes a first sweep generator-deflection amplifiercombination 21, 22 for supplying signals to plates 15 of tube 11 todeflect the electron beam in a horizontal direction. Means 20 alsoincludes a second sweep generator-deflection amplifier combination 23,24 for supplying signals to plates 16 of tube 11 to deflect the electronbeam in a vertical direction. The raster developed by these deflectionsignals scans the target 13 in the well known manner. As was previouslymentioned, the size and centering of the raster may vary as either ofthe electrode potentials B and B vary and/ or as the spatial orientationof either of the plates of the electrostatic deflection plate pairs 15and 16 vary and/or as the concentricity of the electron beam enteringthe deflection field varies. Such variations in the size and centeringof the raster generally reduces the usefulness of the imaging system.

The imaging system additionally includes means associated with the imagetube 11 for producing an electrical signal in response to the variationsin the size and centering of the raster. This means includes a reticleused to form a frame bordering the electrical image stored on thephotosensitive target 13. The reticle may be part of the image tube 11,as where it is engraved onto its face plate 19, or it may be external tothe image tube 11, as where it is held either in front of the face plate19, or attached thereto, or imaged thereon. In either case, however, thereticle is imaged onto the target 13 to form the frame. Anotherarrangement which may be used is one where the reticle is engraved ontothe target 13 itself, in which case, the reticle and the frame will beone and the same. In any event, the configuration of the frame will bethe same as the configuration of the reticle. The reticle dimensions areso chosen that when the frame is formed on the target 13 the frame willbe overlapped by the raster and will produce an electrical signalindicative of the overlapping. As the nature and extent of theoverlapping of the frame by the raster varies, the electrical signalsproduced by the frame also varies.

The imaging system further includes means re- 2 sponsive to theelectrical signals produced by the frame for deriving a plurality ofcontrol signals representative of the size and centering variations ofthe raster. Means .30 includes comparator circuits 31, 32, 33, and 34which develop a horizontal size control signal, a horizontal centeringcontrol signal, a vertical size control signal, and a vertical centeringcontrol signal, respectively. Circuit 31, includes a two input ANDcircuit 35, an adder circuit 36, and an integrator circuit 37, connectedin series and in the order mentioned. Circuit 32 includes a two inputAND circuit 38, an inverter circuit 39, an adder circuit and anintegrator circuit 41, also connected in series and in the ordermentioned. Circuit 33 includes a two input AND circuit 42, an addercircuit 43, and an integrator circuit 44, connected in that order.Circuit 34 includes a two input AND circuit 45, an inverter circuit 46,an adder circuit 47, and an integrator circuit 48, also connected inthat order.

Means 30 also includes a synchronizing circuit connccted to thecomparator circuits 3134 for supplying enabling and reference pulses tobe used by the units therein in developing the size and centeringcontrol signals. Circuit 50 is also connected to sweep generators 21 and23 for supplying timing signals used to synchronize the horizontal andvertical scan of the image tube 11 to the horizontal and vertical scanof the television receiver 55. A block diagram of such a synchronizingcircuit is shown in FIG. 9.

The imaging system finally includes means, such as conductors 61-64, forcoupling the control signals developed by means 30 to the deflectionsignal supply means 20 to prevent the variations of the raster.Conductor 61 couples the control signal developed at the output ofintegrator 37 to the sweep generator 21 to prevent any variations in thehorizontal size of the raster. Conductor 62 couples the control signaldeveloped at the output of integrator 41 to the deflection amplifier 22to prevent any variations in the horizontal centering of the raster.Conductor 63 couples the control signal developed at the output ofintegrator 44 to the sweep generator 23 to prevent any variations in thevertical size of the raster. Conductor 64 couples the control signaldeveloped at the output of integrator 43 to the deflection amplifier 24to prevent any variations in the vertical centering of the raster.

The video signal produced as the raster scans across the target 13, or,more particularly, the video signal produced as the raster scans acrossthe electrical image stored on the target 13, that image being a replicaof the optical image of the scene to be televised 100, is coupled fromtarget ring 14 through bandpass amplifier 70, envelope detector 71,dilferentiator 72, and bistable multivibrator 73 to one input of each ofthe AND circuits 35, 38, 42, and 45. The video signal is also coupled totelevision receiver 55.

The operation of the imaging system will be described with reference tothe manner in which the variations in the horizontal size and horizontalcentering of the raster are prevented. The variations in the verticalsize and vertical centering of the raster are prevented in a likemanner. In considering the operation, it will be assumed that thereticle is attached to the face plate 19 of image tube 11 (notation inFIG. 1). Reticle 80 may be a thin piece of glass having an index ofrefraction equal to the index of refraction of the face plate 19 and iscemented thereto with a matching index of refraction cement. Reticle 80may be a grating of equally spaced vertical lines which is imaged ontothe target in such a manner that the imaged grating lines areperpendicular to the direction of the horizontal scan of the raster. Theconfiguration of the frame 80' (i.e., the configuration of the reticleimaged onto target 13) and the alignment of the grating lines of theframe 81' with one line of the raster 82 is shown in FIG. 2a.

The operation of the system can best be described with reference to thesignal waveforms a-l shown in FIGS.

3, 4, and 5. FIG. 3 shows the signal waveforms a-l present at differentpoints in the imaging system when the horizontal size and horizontalcentering of the raster are correct. FIG. 4 shows the waveforms for thecase where the raster is oversized and displaced to the left of thecenter of the frame 33. FIG. 5 shows the waveforms for the case wherethe raster is undersized and displaced to the right of the center of theframe, 83. In the discussion that follows, it will be assumed that thecenter of the frame 83, coincides with the center of the electricalimage stored on target 13. To aid in the understanding of the invention,letter notations have been placed at various points in the imagingsystem of FIG. 1, at which points the signal waveforms of FIGS. 35having the same letter notations appear.

In operation, assume that the horizontal size and centering of theraster are correct (FIG. 3). For this condition the raster scans acrossan equal number of grating lines both on the left and right hand sidesof the frame 80'. Electrical signals are produced in response to thescanning by the raster of the spacing between the lines of the grating81 but no electrical signals are produced in response to the scanning bythe raster of the lines of the grating themselves. The video signalappearing at the target ring 14 for the correct size-correct centeringcondition of the raster is shown by waveform a in FIG. 3portion Q) isthe video signal produced as the raster scans across the grating lineson the left-hand side of the frame, i.e., at the start of the scan;portion is the video signal produced as the raster scans between theleft and righthand sides of the frame 8% and which is to be displayed byreceiver 55 (the desired areas of the electrical image stored on target13); and portion is the video signal produced as the raster scans acrossthe right-hand side of the frame 86, i.e., at the end of the scan.Portions represent the retrace intervals of the video signal.

The frequency of the video signal produced as the raster scans acrossthe frame 80 (the frequency of the signal portions and depends on thespacing of the grating lines and on the rate of the scan. The framesignal portions and may be separated from the video signal portion by abandpass amplifier 70. The frame signals thus separated are envelopedetected by unit 71, differentiated by unit 72, and impressed uponmnltivibrator unit 73 which reproduces the waveform generated by theraster scan (FIG. 3, b). The pulses developed by the multivibrator 73are then impressed upon one input of the two input AND circuits and 38.Synchronizing circuit 59 snppiies a start enable pulse (FIG. 3, c) tothe second input of AND circuit 35, and an end enable pulse (FIG. 3, e)to the second input of AND circuit 38. The leading edge of the startenable pulse is in time synchronism with the start of the scan while thetrailing edge of the end enable pulse is in time synchronism with theend of the scan. The time duration of these pulses is equal to twice theduration of the normal pulse developed by multivibrator 73, that pulsebeing developed when the scan size is correct.

The output pulse signal of AND circuit 35 (PEG. 3, d) represents theoverlapping of the frame 80 at the start of the raster scan while theoutput pulse signal of AND circuit 38 (FIG. 3, f) represents theoverlapping of the frame 89' at the end of the raster scan. These twopulse signals, along with a horizontal size reference pulse developed bysynchronizing circuit Si (FIG. 3, g), are supplied to adder 3-5, theoutput signal of which (FIG. 3, h) is supplied to integrator 37. Theduration of the size reference pulse (FIG. 3, g) is such that for theraster condition assumed, the sum of the video pulse energies within thestart and end frame pulse widths equals the energy of the size referencepulse but is of opposite polarity. The output of integrator 37 for thecorrect size condition is therefore 0 volts (FIG. 3, i).

The output pulse signal of AND circuit 38 (FIG. 3, f) is also applied toinverter 39 wherein it is reversed in polarity, the output pulse signalbeing shown as j in FIG. 3. This pulse along with the output pulsesignal of AND circuit 35 (FIG. 3, d), is supplied to adder 40, theoutput signal of which (FIG. 3, k) is supplied to integrator 41. For theraster condition assumed, the video pulse energy within the start framepulse Width equals the video pulse energy within the end frame pulsewidth so that the output of integrator 41 for the correct centeringcondition is also 0 volt (FIG. 3, I).

When the scan size increases above normal (FIG. 4 for example), theraster overlaps more of the frame both on its left and right-hand sides.The video pulse energy within the start and end frame pulse widthsincrease (FIG. 4, d and 4, 1) but the energy of the size reference pulseremains unchanged (FIG. 4, g). As a result, integrator 37 develops apositive D-C signal at its output (FIG. 4, i). This signal is applied tosweep generator 21 to decrease the amplitude of the deflection signalsupplied to the plate 15 to decrease the size of the raster.

When the scan size decreases below normal, (FIG. 5 for example), theraster overlaps less of the frame 80' both on its left and right-handsides. The video pulse energy within the start and end frame pulsewidths decrease (FIGS. 5, d and 5, but the energy of the size referencepulse again remains unchanged (FIG. 5, g). As a result, integrator 3'7develops a negative D-C signal at its output (FIG. 5, i). This signal isapplied to sweep generator 21 to increase the amplitude of thedeflection signal supplied to the plates 15 to increase the size of theraster.

When the center of the raster drifts towards the left of the center ofthe frame, 83 (FIG. 4, for example), the raster overlaps more of theleft-hand side of the frame 80 than it does the right-hand side. Thevideo pulse energy within the start frame pulse width (FIG. 4, d)

therefore increases with respect to the video pulse energy within theend frame pulse width (FIG. 4, 1). As a result, integrator 41 develops apositive D-C signal at its output (FIG. 4, I). This signal is applied todeflection amplifier 22 to vary the differential D-C value of thedefiection signal supplied to the plates 15 in a direction to shift thecenter of the raster towards the right, i.e., in a direction to recenterthe raster.

When the center of the raster drifts toward the right of the center ofthe frame, 83 (FIG. 5 for example), the raster overlaps more of theright-hand side of the frame, 821* than it does the lefthand side. Thevideo pulse energy within the start frame pulse Width (FIG. 5, d)therefore decreases with respect to the video pulse energy within theend frame pulse width (FIG. 5, 1). As a result integrator 41 develops anegative D-C signal at its output (FIG. 5, I). This signal is applied todeflection amplitier 22 to vary the differential D-C voltage of thedeflection signal supplied to the plates 15 in a direction to shift thecenter of the raster towards the left, i.e., in a direction to recenterthe raster.

The polarities of the D-C control signals developed by integratorcircuits 37 and 41 for all conditions of horizontal size and centeringvariations are given in Table I below:

The vertical size and vertical centering control signals are developedin a manner similar to the manner in which the horizontal size andhorizontal centering control signals are developed. The polarities ofthe control signals det ssazet 7 veloped by the integrator circuits 44and 48 for different conditions of vertical size and centering aresummarized in Table II below:

Referring to FIG. 2!), there is shown another form of frame (imagedreticle) 90' which may be used to produce an electrical signalindicative of the variations in the size and centering of the raster.The actual reticle may, as before, be a thin piece of glass cemented tothe face plate 19 of image tube 11 with a matching index of refractioncement. Frame 90' differs from frame 80 in that whereas rame 80'consisted entirely of a vertical grating borderirig that portion of theelectrical image which is to be displayed, frame 90' consists of acomposite black and white band (91' and 92', respectively) whichtogether border that portion of the electrical image which is to bedisplayed. The alignment of bands 91' and 92 with one line of the raster82 is also shown in FIG. 2b. Electrical signals are produced in responseto the scanning by the raster of the white band 92 but no electricalsignals are i produced in response to the scanning by the raster of theblack band 91'. Thus, when the raster scans across the target 13, andmore particularly, when the raster scans across the frame 94) and thestored electrical image, a video signal is produced having the generalwaveform shown in FIG. 6, waveform a for example-portion [D is the videosignal produced as the raster scans across the left-hand side of thewhite band 92, i.e., at the start of the scan; portion (2) is the zerolevel produced as the raster scans across the left-hand side of theblack band 91'; portion is the video signal produced as the raster scansbetween the leftand right-hand sides of the black band 91 and which isto be displayed by receiver 55 (the desired areas of the electricalimage stored on target 13) portion (4) is the zero level produced as theraster scans across the right-hand side of the black band 91'; andportion 6) is the video signal produced as the raster scans across theright-hand side of the white band 92, i.e., at the end of the scan.Portions (9 represent the retrace intervals of the video signal.

Referring back to waveform a in FIGS. 3-5, for a moment, it will benoted therefrom that there is no time separation between the signalproduced as the raster scans across the frame 80' (portions (D and andthe signal produced as the raster scans between the left and right-handsides of the frame (the signal to be displayed, portion In order to usethe frame signals to develop the size and centering control signals, itwas first necessary to separate the frame Signals from the signal to bedisplayed in bandpass amplifier 70. Referring to waveform a in FIG. 6,however, it will be noted that the video signals produced at the ends ofthe raster (portions (D and 6)) are already separated from the videosignal to be displayed (portion by the black band portions and Thus, thevideo signal produced as the raster scans across the target 13 may becoupled directly from the target ring 14 to the AND circuits 35, 38, 42and 45. In other words, by using the combination black and white bandreticle instead of the vertical grating reticle, units 70-73 may beomitted from the imaging system of FIG. 1. It is apparent thereforethat, whereas the imaging system using a vertical grating reticleutilizes both frequency and time separation for the size and centeringcontrol signals, the imaging system using the combination band reticleutilizes time separation only.

FIGS. 6, 7, and 8 and the waveforms a-l therein show the signalsdeveloped at the same points in the imaging system of FIG. 1 as werepreviously used for the correct sizecorrect centering condition,oversizeddisplaced left centering condition, and undersizeddisplacedright centering condition of the raster, respectively.

For the normal size-normal centering condition shown in FIG. 6, theraster scans beyond both sides of tht black band 91 into the White bandportion, a distance equal to the width of the black band. Since the rateof scan is constant along the length of the raster, the time durationsof portions Q), and in Waveform a of FIG. 5 are all equal. The leadingedge of the start enable pulse supplied by unit 59 is in timesynchronism with the start of the scan while the trailing edge of theend enable pulse is in time synchronism with the end of the scan. "thetime duration of both the start enable and end enable pulses is 50%greater than the normal time duration of portions (D, or C The size andcentering control signals are derived from integrator circuits 37, 41,44 and 48 in the previously described manner. The relationshipsexpressed in Tables I and II for the vertical grating reticle also applyfor the black and white band reticle.

To summarize then, for the correct raster size condition the sum of thevideo pulse energies within the start and end frame pulse widths(grating pulse widths or white band pulse widths) is just cancelled bythe energy of the size reference pulse thereby producing zero voltsoutput. When the raster size decreases below normal, the sum of thevideo pulse energies within the start and end frame pulse widthsdecrease below the energy of the size reference pulse, thereby producinga negative D-C signal output which is used to increase the respectivedeflection size. When the raster size increases above normal, the sum ofthe video pulse energies within the start and end frame pulse widthsincreases above the energy of the size reference pulse, therebyproducing a positive D-C signal output which is used to decrease therespective deflection size. When the raster centering drifts toward thestart side of the scan, the video pulse energy within the start framepulse width increases with respect to the video pulse energy within theend frame pulse width, thereby producing a positive D-C signal outputwhich is used to shift the raster toward the end side of the scan. Whenthe raster centering drifts toward the end side of the scan, the videopulse energy within the start frame pulse width decreases with respectto the video pulse energy within the end frame pulse width, therebyproducing a negative D-C signal output which is used to shift the rastertowards the start side of the scan.

It will be evident from the foregoing description that the image tube isnow included within the feedback loop. As a result of this inclusion,and as a result of using a physical reftrence in the form of a reticle,the size and centering of the scanning raster will be maintainedconstant, independent of system variations whether electrical,electro-mechanical, or mechanical and whether within or external to theimage tube. Thus the raster will accurately scan that area of the storedelectrical image which is to be displayed.

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention and it is, therefore, aimedto cover all such changes and modifications as fall within the truespirit and scope of the invention.

What is claimed is:

1. An imaging system comprising:

means for converting an electromagnetic image into an electrical signalrepresentative of the image by converting the electromagnetic image intoan electrical image and by sequentially scannin the electrical image;

means for supplying deflection signals to the converting means todevelop a raster used to perform said sequential scanning, the raster sodeveloped having at least one dimensional characteristic which may varyundesirably;

means for providing as part of said electrical image a frame in the formof a reticle bordering desired areas of the electrical image, said framebeing adapted to be in an overlapping relationship with at least oneside of the raster;

means for deriving from the electrical signal representative of theimage electrical signals indicative of the extent of overlapping of saidframe by the raster;

comparison means utilizing the signals indicative of the extent ofoverlapping of said frame by the raster for developing a plurality ofcontrol signals representative of the variations of the raster;

and means for coupling the control signals to the deflection signalsupply means to prevent said variations so as to provide accuratescanning of the desired areas of the electrical image by the raster.

2. An imaging system comprising:

means for converting an optical image into an electrical image whichwhen sequentially scanned produces an electrical signal output inresponse thereto;

means for supplying deflection signals to the converting means todevelop a raster used to scan the stored electrical image, the raster sodeveloped having at least one dimensional characteristic which may varyundesirably;

means for providing as part of said electrical image a frame in the formof a reticle bordering desired areas of the electrical image, said framebeing adapted to be in an overlapping relationship with at least oneside of the raster;

means for deriving from the electrical signal representative of theimage electrical signals indicative of the extent of overlapping of saidframe by the raster;

comparison means utilizing the signals indicative of the extent ofoverlapping of said frame by the raster for developing a plurality ofcontrol signals representative of the variations of the raster;

and means for coupling the control signals to the deflection signalsupply means to prev nt said variations so as to provide accuratescanning of the desired areas of the electrical image by the raster.

3. An imaging system comprising:

image sensor means for converting an optical image into a storedelectrical image which when sequentially scanned produces an electricalsignal in response thereto, said means including an electron gun and aphotosensitive target toward which the electron beam produced from saidgun is directed and upon which is stored the electrical image;

means for supplying deflection signals to the sensor means to develop ascanning raster from said beam, the raster so developed having at leastone dimensional characteristic which may vary undesirably;

means for providing as part of said electrical image a frame in the formof a reticle bordering desired areas of the electrical image, said framebeing adapted to be in an overlapping relationship with at least oneside of the raster;

means for deriving from the electrical signal representative of theimage electrical signals indicative of the extent of overlapping of saidframe by the raster;

comparison means utilizing the signals indicative of the extent ofoverlapping of said frame by the raster for developing a plurality ofcontrol signals representative of the variations of the raster;

and means for coupling the control signals to the deflection signalsupply means to prevent said varia tions so as to provide accuratescanning of the desired areas of the electrical image by the raster.

4. An imaging system comprisin image sensor means for converting anoptical image into a stored electrical image which when sequentiallyscanned produces an electrical signal in response thereto, said meansincluding a vidicon type image tube having an electron gun, aphoto-sensitive target toward which the electron beam produced from saidgun is directed and upon which is stored the electrical image, anaccelerating electrode, a de celerating electrode, a pair of horizontalelectrostatic deflection plates, and a pair of vertical electrostaticdeflection plates:

means for supplying first deflection signals to said horizontalelectrostatic plates and second deflection signals to said verticalelectrostatic plates to develop a scanning raster from said beam, theraster so developed having a tendency to vary undesirably in size andcentering as the deflection signal supplied and the electrode supplyvoltages, vary, said means including a first deflection amplifier-sweepgenerator combination for supplying the horizontal deflection ignals anda second deflect-ion amplifier-sweep generator combination for supplyingthe vertical signals;

means associated with the image sensor means for producing an electricalsignal in response to the variations of the raster, said means includinga reticle cemented onto the faceplate of the vidicon type image tube andwhich when imaged onto the target of said tube forms a frame borderingdesired areas of the stored electrical image, said frame being adaptedto be in an overlapping relationship with at least one side of theraster;

means responsive to the electrical signal produced by saidlast-mentioned mean sincluding a first comparator circuit having a sumsignal channel for deriving a first control signal representative of thevariations in the horizontal size of the raster, a second comparatorcircuit also having a sum signal channel for deriving a second controlsignal representative of the variations in the vertical size of theraster, a third comparator circuit having a difference signal channelfor deriving a third control signal representative of the variations inthe horizontal centering of the raster, and a fourth comparator circuitalso having a difference signal channel for deriving a control signalrepresentative of the variations in the vertical centering of theraster;

and means for coupling the horizontal size control signal to the sweepgenerator of the first deflection amplifier-sweep generator combination,for coupling the vertical size control signal to the sweep generator inthe second deflection amplifier-sweep generator combination, forcoupling the horizontal centering control signal to the deflectionamplifier in the first deflection amplifier-sweep generator combination,and for coupling the vertical centering control signal to the deflectionamplifier in the second deflection amplifier-sweep generator combinationwhereby the size and centering variations of the scanning raster causedby variations in the deflection signals and by variations in theelectrode supply voltages are prevented.

5. An imaging system according to claim 1 in which said frame is adaptedto be in an overlapping relationship with the two sides of the rasterparallel to a first axis and in which the comparison means includes afirst comparator circuit for comparing signals representative of theextent of overlapping along a second axis, orthogonal to said firstaxis, with a signal representative of the desired extent of overlappingalong said second axis for deriving control signals representative ofthe size of the raster along said second axis.

6. An imaging system according to claim 1 in which said frame is adaptedto be in an overlapping relationship with the four sides of the rasterand in which said comparison means includes a first comparator circuitfor comparing the Signals representative of the extent of overiappinalong a x coordinate axis with a signal indicative of the desired extentof overlap along the x coordinate axis and a second comparator circuitfor comparing the signals representative of the extent of overlappingalong a y coordinate axis with a signal indicative of the desired extentof overlapping along the y coordinate axis for deriving control signalsrepresentative of the x coordinate and y coordinate scan size of theraster.

7. An imaging system according to claim 1 in which said frame is adaptedto he in an overlapping relationship with at least the two sides of theraster parallel to a first axis and in which the comparison meansincludes a first comparator circuit for comparing signals representativeof the extent of overlapping of the raster at opposite ends of saidfirst axis for deriving control signals representative of the first axiscentering of the raster.

8. An imaging system according to claim 1 in which said frame is adaptedto be in an overlapping relationship with the four sides of the rasterand in which the comparison means includes a first comparator circuitfor comparing signals representative of the extent of overlapping of theraster at opposite ends of a x coordinate axis and a second comparatorcircuit for comparing signals representative of the extent ofoverlapping at opposite ends of a y coordinate axis for deriving controlsignals representative of the x coordinate and y coordinate centering ofthe raster.

9. An imaging system according to claim 1 in which said frame is adaptedto be in an overlapping relationship with the two sides of the rasterparallel to a first axis, and in which the comparison means includes afirst comparator circuit for comparing signals representative of theextent of overlapping along a second axis, orthogonal to said firstaxis, with a signal representative of the desired extent of overlappingalong said second axis and a second comparator circuit for comparingsignals representative of the extent of the overlapping of the raster atopposite ends of said second axis for deriving control signalsrepresentative of the second axis size and centering of the raster.

10. An imaging system according to claim 1 in which said frame isadapted to be in an overlapping relationship with the four sides of theraster and in which the comparison means includes a first comparatorcircuit for comparing the signals representative of the extent ofoverlapping of a x coordinate axis with a signal indicative of thedesired extent of overlapping along the x coordinate axis, a secondcomparator circuit for comparing the signals representative of theextent of overlapping along a y coordinate axis with a signal indicativeof the desired extent of overlapping along the y coordinate axis. athird comparator circuit for comparing the signals representative of theextent of overlapping of the raster at the opposite ends of the xcoordinate axis and a fourth comparator circuit for comparing thesignals representative of the extent of overlapping at opposite ends ofthe y coordinate axis for deriving control signals representative of thex coordinate and y coordinate size and centering of the raster.

11. An imaging system according to claim 10 in which the frame borderingdesired areas of the electrical image is in the form of a grating whichproduces an electrical signal in response to the overlapping by theraster of the spacing between the lines of the grating and whichproduces no electrical signals in response to the overlapping by theraster of the lines of the grating themselves.

12. An imaging system according to claim 10 in which the frame borderingdesired areas of the electrical image is in the form of a combined bandhaving a first portion which produces electrical signals when overlappedby the raster and having a second portion which produces no electricalsignals when overlapped by the raster.

13. An imaging system according to claim 3 in which said frame isadapted to be in an overlapping relationship with the two sides of theraster parallel to a first axis and in which the comparison meansincludes a first comparator circuit for comparing signals representativeof the extent of overlapping along a second axis, orthogonal to saidfirst axis, with a signal representative of the desired extent ofoverlapping along said second axis for deriving control signalsrepresentative of the size raster along said second axis.

14. An imaging system according to claim 3 in which said frame isadapted to be in an overlapping relationship with at least the two sidesof the raster parallel to a first axis and in which the comparison meansincludes a first comparator circuit for comparing signals representativeof the extent of overlapping of the raster at opposite ends of saidfirst axis for deriving control signals representative or the first axiscentering of the raster.

References Cited UNITED STATES PATENTS 2,613,263 l0/l952 Hilburn 1787.23,126,447 13/1964 Bendell 1785.4 3,182,224 5/1965 Stone et al. 315213,210,597 lO/1965 Siegmund et a1. 3l521 FOREIGN PATENTS 776,764 6/1957Great Britain.

ROBERT L. GRIFFIN, Primary Exalrzirzer. JOHN W. CALDWELL, Examiner. R.K. ECKERT, Assistant Examiner.

